First, to to Setup->Playback Engine...
At the top, select Digidesign Eleven Rack (if you're using another piece of hardware, e.g. Digi 002R, you'd select that, of course)
Click OK.
Go to the track you want to arm.
Whatever you had selected as the input, e.g. Eleven Rig L/R, will still be disabled, so select another input.
Now select the input you want, which should appear as enabled now.
I hope this helps.
I'm not a huge fan of PT, but, I've had this issue before and it's easily fixed.